1. Field of the Invention
The present invention relates to a novel steam turbine rotor having an attachment structure with respect to an inverted fir-tree blade root which is inserted in the axial direction, and to a novel inverted fir-tree turbine blade. The present invention also relates to a low-pressure steam turbine with those rotors and blades, and to a steam turbine power plant with those turbines.
2. Description of the Related Art
From the viewpoint of realizing higher capacity and higher efficiency of a steam turbine, one of the most important themes is to obtain a longer blade in the last stage of a low-pressure steam turbine. To be adapted for a centrifugal force increased with the longer blade in the last stage of the low-pressure steam turbine, design has been generally conducted aiming to increase the material strength. However, a rotor material, in particular, has higher sensitivity to stress corrosion cracking (SCC) with an increase of the material strength, and the material strength of the rotor cannot be so increased as that of a blade material. Accordingly, the longer blade in the last stage tends to increase the difference in material strength between the blade material and the rotor material which are practically usable, and to reduce a margin for the allowable stress in the rotor. That tendency gives rise to a technical problem in point of how to take stress balance between the blade and the rotor.
The related art for a turbine blade in consideration of the material difference between the blade and the rotor is disclosed in, e.g., Patent Document 1 (JP,A 60-65204). Patent Document 1 discloses a structure in which, taking into account bending of a blade hook and a rotor hook in the direction of a contact surface, the thickness of each hook of the blade and the rotor is selected in reverse proportion to the longitudinal elastic modulus of the material, thereby reducing unbalance contact and avoiding concentration of stresses.
Also, Patent Document 2 (JP,A 5-86805) discloses an inverted fir-tree turbine blade having a neck structure in which the upper radius is larger than the under radius in a blade neck at the outermost circumference. Patent Document 3 (JP,A 6-108801) and Patent Document 4 (JP,A 63-306208) disclose inverted fir-tree turbine blades having particular hook and neck structures.